Most digital systems transmit a bipolar-coded signal, or some code modification such as bipolar with constraints for zero suppression, to virtually eliminate DC and low-frequency signal components. In telecommunication systems incorporating digital transmission, this mitigation of low-frequency energy permits, for example, transformer coupling between transmission circuitry and repeaters. However, the alternate polarity signal is pseudoternary in the sense that only two bits of information may be conveyed wherein three bits are available in each time slot. Thus, the penalty incurred is approximately a sixty percent [(log 3/log 2-1] loss of bit rate capacity.
The system disclosed in U.S. Pat. No. 4,071,692 issued Jan. 31, 1978 to Weir et al, provides some increase in capacity but at the expense of increased low frequency energy. In this reference, a PCM system is described wherein the intelligence is conveyed via a bipolar signal with zero suppression. To provide some additional information (typically synchronizing pulses) when needed, violations of the alternating bit rule are imposed. To maintain a low DC level, a pseudo-random bit stream is added to the intelligence signal prior to coding, thereby causing numerous bit transitions that mask the violations. Even with this provision for additional information transfer, there is still a significant loss of capacity from the theoretical since the increase amounts to only one additional bit per frame.